1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) communication system, and more particularly to a method of performing an AFC (Automatic Frequency Control) process in a mobile station in such CDMA communication system.
2. Description of the Related Art
Recent years have seen much attention to a CDMA communication system that is highly resistant to interferences and disturbances as a mobile communication system. In the CDMA communication system, a transmitting station spreads a user signal to be transmitted with a spreading code and transmits the spread signal, and a receiving station despreads the spread signal using a spreading code which is the same as the spreading code used by the transmitting station for thereby obtaining the original user signal.
The receiving station cannot despread the spread signal unless the spreading codes used in the transmitting and receiving stations are synchronized in phase with each other. To achieve the spreading code synchronization, mobile stations of the CDMA communication system use a TCXO (Temperature Controlled Xtal Oscillator) of very high frequency accuracy as a reference oscillator for generating a reference frequency signal for use in modulating signals received from a base station, and perform an AFC process for equalizing the frequency of the reference frequency signal to the frequency of a reference frequency signal of the base station which serves as a transmitting station.
The AFC process is carried out based on a pilot symbol contained in data that are transmitted from the base station to the mobile station.
The physical format of a down link for data transmission from a base station to a mobile station will be described below with reference to FIG. 1 of the accompanying drawings.
Data transmitted from the base station comprises a plurality of radio frames 31 each having an interval of 10 ms. Each of the radio frames 31 comprises 16 time slots 321 through 3216. Each of the time slots 321 through 3216 contains pilot symbols 33. While the pilot symbols 33 have different values depending on the time slots 321 through 3216, they have a predetermined pattern. Therefore, the mobile station is capable of recognizing pilot symbols to be transmitted before receiving the pilot symbols. There are four pilot symbols contained in each time slot. The mobile station can measure a frequency error with respect to the base station using the four pilot symbols per time slot.
The frequency error will be described below with reference to FIG. 2 of the accompanying drawings. In the CDMA communication system, QPSK (Quadrature Phase Shift Keying) is employed as a primary modulation process to be effected on data before the data is spread, and hence each symbol comprises 2-bit data, i.e., each symbol takes a value of (0, 0), (0, 1), (1, 0), or (1, 1). These values as plotted in a vector-diagram are shown in FIG. 2. In FIG. 2, the horizontal axis represents the magnitude of an In-phase component (I) and the vertical axis the magnitude of a Quadrature component (Q). In FIG. 2, a frequency error between measured data 91 of a first pilot symbol and measured data 90 of a second pilot symbol is xcex8.
Immediately after a mobile station in a CDMA mobile communication system is turned on, the mobile station performs an AFC process using a BCCH (Broadcast Control Channel). When the frequency error between a reference frequency and the frequency of a signal received from the base station becomes smaller than a certain value, the mobile station determines the frequency as locked, and terminates the AFC process.
Operation of the mobile station after it has been turned on and until it enters a waiting mode will be described below with reference to FIG. 3 of the accompanying drawings. When the mobile station is turned on, the mobile station receives all BCCHs which can be received in step 201, and identifies a base station which is transmitting the BCCH whose field intensity is greatest, i.e., whose S/N ratio is best, of all the received BCCHS, in step 202. A BCCH is a one-way control channel for broadcasting control information from the base station to the mobile station.
In step 203, the mobile station transmits information such as of an ID number of the base station to the base station via a RACH (Random Access Channel). The base station transmits various items of information to the mobile station via a FACH (Forward Access Channel) in step 204.
The mobile station stores the received information, and enters a waiting mode for receiving a PCH (Paging Channel) from the base station in steps 205, 206.
If the mobile station is notified of an incoming call via the PCH in step 206, then the mobile station receives a BCCH to obtain information again from the base station in step 207. In step 208, the mobile station receives a DTCH (Data Channel) to begin speech communications. When the speech communications end, the mobile station enters the waiting mode again in steps 205, 206.
In step 201 or 202 immediately after the mobile station is turned on, the mobile station performs the AFC process using the received BCCH. Once the frequency is locked in the AFC, the mobile station does not perform the AFC process again.
However, while the mobile station is in speech communication with the base station via the DTCH, the frequency of the reference frequency signal outputted from the reference oscillator may fluctuate due to the heat generated by an amplifier which generates transmission power. Furthermore, if the speech communication mode continues for a long period of time, then the reference frequency signal may fluctuate owing to changes in the ambient temperature of the mobile station. As a consequence, an error between the reference frequency of the base station and the reference frequency of the mobile station becomes greater than a certain value due to a change in the frequency of the reference frequency signal, so that the mobile station tends to fail to properly despread data transmitted from the base station.
It is therefore an object of the present invention to provide a method of performing an AFC process in a mobile station for preventing received data from failing to be despread even if the frequency of a reference oscillator fluctuates in a speech communication mode and even if the phase varies due to fading.
To achieve the above object, in accordance with an aspect of the present invention, there is provided a method of performing an automatic frequency control process in a mobile station to equalize the frequency of a reference frequency signal for use in demodulating a signal received from a base station, with the frequency of a reference frequency signal in the base station. The automatic frequency control process is carried out in a speech communication mode in which a data channel transmitted from said base station is being received.
According to another aspect of the present invention, the automatic frequency control process is carried out using a broadcast control channel or a data channel in a speech communication mode in which the data channel transmitted from said base station is being received.
According to still another aspect of the present invention, if a frequency error measured using a data channel transmitted from said base station is greater than a predetermined value, said automatic frequency control process is carried out using a broadcast control channel in a speech communication mode in which said data channel is being received.
With the above arrangements, since the AFC process is carried out using the data channel or the broadcast control channel even in the speech communication mode in which the data channel is being received, no reception failure occurs even when a reference oscillator suffers frequency fluctuations in the speech communication mode.
According to yet another aspect of the present invention, said automatic frequency control process is carried out using a broadcast control channel when the broadcast control channel is received as instructed by said base station in a speech communication mode in which a data channel transmitted from said base station is being received.
With the above arrangement, inasmuch as the AFC process is carried out using the broadcast control channel when the broadcast control channel in the speech communication mode in an ordinary processing sequence, the AFC process can be performed without the need for no special processing for receiving the broadcast control channel.
The AFC process may be carried out by controlling a reference oscillator which generates the reference frequency signal.